Further investigation revealed that packstack had been upgraded in the interim (via yum upgrade on the controller). I had to downgrade packstack so that the version which originally generated the answer file is the same as the version I'm using now.

In my case, this is the 0.1 version listed above. I downgraded all three packages to 0.1. Be a little careful, the 0.8 and the 1.0 versions were released the same day so downgrading to 0.8 did nothing for me. After downgrading to 0.1, I stopped getting the messages and packstack ran to completion.

Another possible solution would be to generate a new answer file with the new version of packstack, and edit it so that the configuration was the same as the original.